Left bundle branch block (LBBB) and right bundle branch block (RBBB) represent common intraventricular conduction delays observed on the surface electrocardiogram. These conditions arise from delayed activation of either the left or right ventricular myocardium due to impaired conduction within the respective bundle branches of the His-Purkinje system. Interpreting these patterns requires understanding the underlying pathophysiology, recognizing characteristic ECG features, and appreciating their implications for patient prognosis and management.
Understanding the Bundle Branches and Conduction
The heart's electrical impulse originates in the sinoatrial node and travels through the atria, reaching the atrioventricular node. It then proceeds down the His bundle, which divides into the right and left bundle branches. These branches rapidly distribute the impulse throughout the ventricular myocardium via the Purkinje fibers, ensuring near-synchronous contraction. A block within one of these branches disrupts this coordinated activation, causing the ventricle supplied by the affected branch to depolarize later than normal, which manifests distinctly on the ECG.
Defining Right Bundle Branch Block (RBBB)
ECG Characteristics and Mechanism
RBBB occurs when the electrical impulse cannot propagate normally through the right bundle branch, delaying right ventricular activation. The primary ECG findings include a wide QRS complex exceeding 120 milliseconds, an rsR' pattern (often termed "rabbit ears") in the right precordial leads (V1-V2), and a wide, slurred S-wave in the left precordial and lateral leads (I, V5, V6). This pattern reflects initial left ventricular depolarization followed by delayed activation of the right ventricle through septal conduction.
Defining Left Bundle Branch Block (LBBB)
ECG Diagnostic Criteria and Physiology
LBBB signifies delayed activation of the left ventricle due to conduction failure in the left bundle branch. The ECG hallmarks include a wide QRS complex greater than 120 milliseconds, a broad monophasic R-wave in leads I, V5, and V6, with the absence of a q-wave in these leads, and a deep, wide S-wave in V1. The ST segments and T waves typically display directional discordance relative to the main QRS deflection, often referred to as "opposite direction" criteria, which helps distinguish LBBB from secondary repolarization abnormalities.
Differentiating LBBB and RBBB Patterns
Practical Analysis on the ECG Strip
Accurate differentiation relies on systematic analysis of the QRS morphology across specific leads. For RBBB, focus on V1: an rSR' complex with a wide terminal R' wave is characteristic. For LBBB, examine V5 and V6: a broad, notched R-wave without an initial q-wave is typical, alongside a dominant S-wave in V1. The axis deviation often seen in RBBB (rightward) versus LBBB (leftward) provides additional supportive evidence, though axis alone is not diagnostic.
Clinical Significance and Associated Conditions
Prognostic Implications and Underlying Causes
While bundle branch blocks can be idiopathic or occur in healthy individuals, they frequently signal underlying cardiac pathology. RBBB is commonly associated with conditions affecting the right ventricle, such as pulmonary hypertension, chronic lung disease, or atrial septal defects. LBBB is more strongly linked to systemic diseases impacting the left ventricle, including hypertension, aortic valve disease, ischemic heart disease, and cardiomyopathies. New-onset bundle branch block, particularly LBBB, may indicate acute myocardial infarction or other significant cardiac events requiring urgent evaluation.
